High-pressure sodium vapor discharge lamp

Abstract: A high-pressure sodium vapor discharge lamp the discharge space of which is enclosed by a ceramic vessel having hermetically sealed ceramic end members with electric feedthroughs connected with electrodes inside the discharge vessel and the discharge vessel contains sodium, noble gas, mercury in a concentration of 0-5 mg/cm³ and at least one further metal additive having a vapor pressure not exceeding 10² Pa at 1000K temperature is characterized in that the molar fraction of sodium in the total metal additive exceeds 0.5 and the molar fraction of sodium is greater than four times the molar traction of mercury.

Description: The invention relates to a high-pressure sodium vapor discharge lamp the discharge space of which is enclosed by a ceramic vessel having hermetically sealed ceramic end members with electric feedthroughs connected with electrodes inside the discharge vessel and the discharge vessel contains sodium, noble gas and equal or less than 5 mg of mercury per cm 3 of the volume of the discharge vessel and at least one further metal additive having a partial vapor pressure not exceeding 10 2 Pa at 1000 K. The operating characteristics of high-pressure sodium lamps are determined by the pressure and composition of the discharge produced in the lamp. As is known, the discharge in the high-pressure sodium lamps contains sodium, mercury and xenon, which have the following characteristic pressures in operation: 10 4 ,10 5 and 3x10 4 Pa, respectively. The required vapor pressures of sodium and mercury are typically ensured by a sodium-mercury amalgam with a weight ratio of 1 to 3. The useful radiation is for the amalgam with a weight ratio of 1 to 3. The useful radiation is for the greatest part provided by the sodium and mercury has the role of increasing the voltage at the lamp terminals, thereby reducing lamp current and making current feedthroughs to be designed easier. One of the significant factors resulting in the market popularity of high-pressure sodium lamps is their long life limited by the voltage rise at lamp terminals during operation. The cause of this voltage rise is the reaction between the sodium content of the lamp and one or more components of the discharge vessel, due to which process sodium will be eliminated from the discharge. This effect is the so-called sodium loss that decreases the molar fraction of sodium in the sodium-mercury amalgam, which, in turn alters the sodium pressure in the discharge. At constant temperature, sodium loss reduces the pressure of sodium which is a minor problem in itself, however, at the same time mercury pressure increases, and with a greater slope than that of the increase of its molar fraction. This latter change will cause the voltage at lamp terminals to rise which will finally result in lamp extinction. The above facts are well-known to those proficient in the field. It is not surprising therefore that several attempts have been made to solve the above problem. The state-of-the-art methods are based on the presumption that the speed of the abovementioned sodium loss is to be slowed. One of the approaches is aimed to reduce the speed of the chemical reaction between the discharge tube wall and sodium, and is described in a study titled "The surface structure of translucent alumina, a scanning electron microscopy investigation" by A.J.H.M. Kock (Proceedings of the symposium on high temperature chemistry II, p: 194-205). Another attempt was to eliminate or reduce the contact of liquid-phase amalgam with the wall as seen in the disclosures of No. GB 2 072 939 and No. HU 181 782 Patent Specifications. All the disclosed approaches - two of which were mentioned as examples only - have proven to be more or less successful, but have been unable to solve the problem of sodium loss. This indicates that the importance of factors affecting the process is still not cleared. In contrast to the earlier efforts, in devising the invention our starting point was that it is the most practical approach which recognizes the fact of sodium loss as a given condition and just strives to compensate for its effects. The invention is based on the recognition that the mercury and sodium vapor pressures prevailing in the conventional high-pressure sodium lamp designs can also be produced using the vapor pressures of even more metals with the additional advantage that when the further additives are chosen appropriately, these can stabilize the luminous efficiency and burning voltage of the plasma, even in the case of sodium loss. In the literature reflecting the recent level of technology, several approaches are found that use, in addition to sodium and mercury, one or more further metal additives in high-pressure sodium lamps. These additives are proposed based on lampmaking considerations. E.g. for the lamp according to Patent Specification No. HU 172 011, in order to simplify the lamp manufacturing process a sodium amalgam resistant to corrosion in atmospheric conditions was prepared using a further metal component. In US Patent No. 4 691 141 an additive dosing method is described according to which the sodium and mercury needed for lamp operation are added in the form of intermetallic compounds formed with a further metal or metals. These compounds are more stable than Na-Hg amalgam and so do not vaporize during sealing the discharge vessel with a frit in a high-temperature furnace. According to US Patent No. 3 521 108 thallium-cadmium is added with the purpose of modifying the lamp spectrum. In the disclosure of US Patent No. 4 639 639 tin, indium or gallium are described as additives, based on the consideration that these can accelerate the warm-up of the lamp, and ultimately produce more favorable operating conditions for the electrodes. An analysis of the known approaches shows the lack of recognition by anyone so far that by adding a third/further metal/metals, a concentration, i.e. additive ratio can be achieved that stabilizes the luminous efficiency of the plasma and the burning voltage, even in the case of sodium loss. In choosing the third/further metal/metals it is a consideration of fundamental importance that its vapor pressure is below 10 2 Pa at the operating temperature of approx. 1000 K, or otherwise it will appear in the discharge during operation and will modify the electrical and photometry parameters of the lamp, an effect intended to be avoided. Furthermore, the metal must not react with the lamp parts including niobium, tungsten, alumina, etc. The important finding of the invention is that the objective - i.e. to stabilize the luminous efficiency of the plasma and the burning voltage, even in the case of sodium loss - is only achieved when the third/further metal/s/ are added so that two requirements are met: the molar fraction of sodium exceeds 0.5 and has a value of at least four times that of the mercury. In order to illustrate the finding of the invention, i.e. to show the efficient concentrations, drawing figures are attached in which: Fig. 1: The concentration range according to the invention shown in the form of a conventional triangle diagram Fig. 2: A further concentration range according to the invention shown in the form of a conventional triangle diagram Fig. 3: A third concentration range according to the invention shown in the form of a conventional triangle diagram Fig. 4: A fourth concentration range according to the invention shown in the form of a conventional triangle diagram. In order to verify the solution according to the invention as described in the Claims, experiments were performed that have completely confirmed our supposition. In the followings the results of such an experiment will be shown as an example for the use of the invention. Example: indium-containing 250W lamps were made using the additives below: "A": 4.6mg sodium + 18.4mg mercury "B": 2.3mg sodium + 18.4mg mercury "C": 4.6mg sodium + 9mg mercury + 12.5mg indium "D": 2.3mg sodium + 9mg mercury + 12.5mg indium. Version "A" represents a conventional high-pressure sodium lamp, while Version "B", a sodium loss of 50%. Version "C" is an embodiment of the invention and Version "D" represents a sodium loss of the same extent as "B" does compared with "A". Obviously, the discharge tube end construction of lamps "C" and "D" had to be slightly modified to ensure that the additives are exposed to a somewhat higher operating temperature corresponding to their composition. Measurements were made on these lamps, the results of which are seen in the following Table: U l (V) P(W) Φ(kLm) η(Lm/W) "A" 90 239 27.3 114 "B" 124 273 30.9 113 "C" 88 232 26.2 113 "D" 110 264 30.0 114 In the Table, U l is the voltage at lamp terminals, P is the lamp wattage, Φ is the luminous flux and η is the luminous efficiency. It is seen in the Table that lamps "A" and "C" have the same data which means that lamps that are equivalent to the conventional ones could be made. The results of measurements performed on lamps "B" show that with the lamp in conventional design a sodium loss of 50% has caused a significant change in luminous and electrical parameters, e.g. the voltage at lamp terminals has increased 34 volts. At the same time, a sodium loss identical with the above has caused only 22 volt rise in lamp "C". All these clearly show that using the finding of the invention resulted in a more stable lamp.

Claims: High-pressure sodium vapour discharge lamp the discharge space of which is enclosed by a ceramic vessel sealed hermetically with ceramic end members which have electric feedthroughs to the feedthroughs electrodes are connected inside the discharge vessel the discharge vessel contains sodium, a noble gas and equal or less than 5 mg of mercury per cm 3 of the volume of the discharge vessel and at least one further metal additive having a partial vapour pressure not exceeding 10 2 Pa at 1000 K characterized in that the molar fraction of sodium in the total metal additive exceeds 0.5 and the molar fraction of sodium is greater than four times the molar fraction of mercury. Discharge lamp according to Claim 1 characterized in that the further metal additive consist of one or more metals selected from the metals Ga, In, Sn, Pb, Bi and Sb. Discharge lamp according to Claim 2 characterized in that the further metal additive is Bi which has the ratio to the sodium expressed by the formula NaBi 0.05-0.25 . Discharge lamp according to Claim 2 characterized in that the further metal additive is Sb which has the ratio to the sodium expressed by the formula NaSb 0.05-0.25 . Discharge lamp according to Claim 2 characterized in that the further metal additive is In which has the ratio to the sodium expressed by the formula NaIn 0.25-1 . Discharge lamp according to Claim 2 characterized in that the further metal additive is Pb which has the ratio to the sodium expressed by the formula NaPb 0.15-0.65 . Discharge lamp according to Claim 2 characterized in that the further metal additive is Sn which has the ratio to the sodium expressed by the formula NaSn 0.15-0.65 .

Inventors: MARTON ZSOLT, MARTON, ZSOLT
Applicants: TUNGSRAM RESZVENYTARSASAG
Application year: 1994
Main classification: H01J61
IPC codes: H01J61